This invention relates to a multi-slice computed tomography (CT) x-ray detector and, more particularly, to a photodiode array activated by backside illumination affording access for electrical connections to individual photodiodes at the opposite, usual front side of the array, while maintaining desired values of the modulation transfer function (MTF) of the detector, enabling the assembly of large two-dimensional detector arrays.
Radiation imaging systems employing such detectors are widely used for medical and industrial purposes, such as for x-ray computed tomography (CT); a typical detector may comprise an array of semiconductor photodiodes, or photodiodes, used to detect light or other ionizing radiation, having attached scintillators. To increase image quality and speed of such detectors, a large number of individual pixels is required. Present technology uses on the order of 1000 to 4000 individual pixels with a respective amplifier per pixel. Some implementations (e.g., GE LIGHTSPEED(trademark) scanners) have configurable detectors wherein plural, respective signal currents from multiple individual photodiodes can be combined for further processing in a single amplifier channel. This arrangement permits the detection area for an individual pixel to be varied, using externally controlled electrical switches. However, as the number of individual amplifier channels and respective pixels is further increased to a desirable number, e.g., xcx9c12000 or more, providing all necessary electrical connections becomes complex and cumbersome.
Present technology uses a single amplifier per photodiode, and thus per pixel, since this affords high data rates and high signal quality. Moreover, present technology provides connections from the photodiodes to the respective amplifiers at the edges of the detector arrays, using a flexible interconnector structure, such as a flexible circuit board (xe2x80x9cflexxe2x80x9d) that brings all of the amplifier connections to edges of the photodiode arrays. However, as the number of amplifier channels increases, the density of the interconnector structures increases to an unattractively high level from the standpoint of complexity, ease of fabrication, and performance. This structure also places some practical fabrication limitations on expanding the area of the array.
It is desirable to provide an imaging device that permits increasing the density of photodiode detection elements in a photodiode array chip and, as well, the total number of photodiodes and the area of the array.
In one representative embodiment, a photodiode detector array is provided that includes a layer of intrinsic semiconductor material having first and second opposite main surfaces, a first doped layer at the first surface of a first conductivity type, and an array of photodiodes on the second main surface comprising respective doped regions of a second conductivity type. The detector array further includes electrical contacts coupled to the second main surface, respectively contacting the doped regions and adapted to convey electrical signals therefrom. Conductors are coupled to the electrical contacts, and a scintillator is optically coupled to the first main surface of the intrinsic semiconductor material. It should be appreciated that intrinsic semiconductor material comprises a lightly doped semiconductor of the first conductivity type, and the use of the term intrinsic describes such a lightly doped semiconductor.
In another representative embodiment, a method of fabricating a photodiode detector array for use in an x-ray detector is provided. The method comprises the steps of forming a layer of intrinsic semiconductor material on a substrate. The layer of intrinsic semiconductor comprises a first surface and a second surface where the first surface is positioned opposite from the second surface. A first doped layer is provided and positioned at the first surface. The first doped layer comprises a first conductivity type. A plurality of second doped regions is provided and positioned at the second surface. The second doped region comprises a second conductivity type. The first conductivity type is opposite to the second conductivity type where the plurality of second doped regions detects radiation incident on the first surface and outputs electrical signals corresponding to the incident radiation. Each of a plurality of electrical contacts is connected to a different one of the plurality of second doped regions. The plurality of electrical contacts extends along the second surface. A first plurality of conductive electrode pads is located on a first board surface of a printed wiring board. Each of the first plurality of conductive electrode pads is aligned with a different one of the plurality of second doped regions, and the printed wiring board is positioned proximate to the second surface. A second plurality of conductive electrode pads is located on a second board surface of the printed wiring board. The second board surface is located opposite from first board surface, and each of the second plurality of conductive electrode pads is connected to a different one of the first plurality of conductive electrode pads. The layer of intrinsic semiconductor material is positioned with the second surface connected to the first board surface, and each of the plurality of electrical contacts is aligned with a different one of the first plurality of conductive electrode pads. A conductive epoxy is applied between each of the plurality of electrical contacts aligned with the first plurality of conductive electrode pads, and each of the plurality of electrical contacts is electrically connected to a different one of the first plurality of conductive electrode pads.